Technological development and increased demand for mobile equipment have led to a rapid increase in the demand for secondary batteries as energy sources. In particular, increased interest in environmental issues has brought about a great deal of research associated with electric vehicles, hybrid electric vehicles and plug-in hybrid electric vehicles which serve as alternatives to vehicles using fossil fuels such as gasoline vehicles and diesel vehicles. These electric vehicles generally use nickel-metal hydride secondary batteries as power sources. However, a great deal of study associated with use of lithium secondary batteries with high energy density and discharge voltage is currently underway and some of them are commercially available.
Such lithium secondary batteries mainly use carbon-based materials as anode active materials and use of lithium metals and sulfur compounds as anode active materials has been considered. Meanwhile, the lithium secondary batteries generally use lithium cobalt composite oxide (LiCoO2) as an anode active material. Also, lithium manganese composite oxides such as LiMnO2 having a layered crystal structure and LiMn2O4 having a spinel crystal structure and lithium nickel composite oxide (LiNiO2) as cathode active materials have been considered.
Lithium nickel oxides such as LiNiO2 are cheaper than the cobalt oxides and exhibit higher discharge capacity when charged to 4.3V. Accordingly, in spite of slightly low average discharge voltage and volumetric density, commercial batteries comprising lithium nickel oxides have improved energy density and a great deal of research on these nickel-based cathode active materials is thus actively conducted in order to develop high-capacity batteries. However, nickel-based cathode active materials have problems of rapid phase transition of crystal structures according to volume variations involved in charge/discharge cycles, rapid deterioration in chemical resistance on the surface thereof exposed to air and humidity and generation of excess gases during storage or cycles. For this reason, nickel-based cathode active materials wherein other transition metals such as manganese or cobalt partially substitute for nickel have been developed. These metal-substituted lithium transition metal compounds advantageously exhibit superior cycle and capacity characteristics.
However, lithium transition metal oxides useful as cathode active materials have disadvantages of low electrical conductivity and insufficient charge/discharge rate characteristics due to low ionic conduction resulting from use of non-aqueous electrolytes.
In order to solve these problems, conventional technologies suggest coating or treating the surface of a cathode active material with a predetermined material. For example, reduction of contact interface resistance between a cathode active material and an electrolyte or by-products formed at a high temperature by coating the cathode active material with a conductive material such as a conductive polymer is well known in the art. However, cathode active materials exerting sufficient battery characteristics have not been developed yet.
Furthermore, high energy density entails the possibility of exposure to extreme risk. Accordingly, as energy density increases, the risk of combustion and explosion disadvantageously increases.
In this regard, a great deal of research according to various approaches is underway, but satisfactory results have not been obtained to date. Improvement in energy density according to increasing complexity and multi-functionalization of mobile equipment has brought about an increase in safety-related problems and there is a need for further improvement in rate characteristics of lithium secondary batteries due to the requirements of electric vehicles, hybrid electric vehicles and power tools.
However, safety and rate characteristics conflict with each other and it is considerably difficult to simultaneously improve these properties and research thereof has not been performed.